The instant invention was designed and developed to meet a need which exists to measure the biomass contained in the tree trunks or the like present in the world's great rain forests. More specifically, the motivation for developing the instant invention is to improve the NASA Global Warming Model. The Global Warming Model predicts the rise or fall of worldwide temperatures using mathematical equations. These equations have major variables, such as the magnitude and extent of the ozone levels, and the so-called "green-house gases" which may prove to impact global temperatures and climate patterns. One of these major variables is the total global amount of embedded CO.sub.2. A large percentage of the embedded CO.sub.2 is contained in the trunks of large trees located in the rain forests of the world.
Previously, there has been no method available for the direct measure of the terrestrial biomass contained in the tree trunks of the rain forests, other than by manual measurements. Manual methods require a person on the ground to measure the diameter and height of each tree. To improve the quality of the global estimation of the quantity of embedded CO.sub.2 it was necessary to develop a technology which would allow for the rapid and remote sensing of the biomass contained in the rain forests world wide and thereby improve the quality of the Global Warming Model. Both the quantity and location of biomass are critical parameters for the modeling of CO.sub.2 flux between the atmosphere and the biosphere for global climate modeling and the prediction of global warming. The measure of terrestrial biomass using conventional ground-based survey methods is difficult, time consuming and labor intensive. Thus, remote sensing is required for the investigation of biomass over the large geographic areas required for global modeling.
Associated with rain forests are thick and dense canopies consisting of small branches and leaves. Optical and infrared sensing systems, which are commonly used for the measure and estimation of certain types of global biomass, cannot see through the rain forest canopies. At optical and infrared frequencies, the energy interacts with the small branches and leaves of the canopy and is reflected back to the sensor. Only the approximate area and the associated biomass of the branches and leaves within direct view can be measured or estimated. The biomass contained in the tree trunks obscured by dense canopies cannot be directly measured or inferred by using currently available electro-optic or infrared sensors.
Recent studies have shown that the currently available Synthetic Aperture Radar (SAR) systems are unable to directly measure vegetative biomass effectively because the radar response saturates at relatively low levels of biomass density. The primary SAR systems previously used for biomass investigation are P-band (.about.500 MHz), L-band (.about.1 GHz), C-band (.about.5 GHz), and X-band (.about.10 GHz). The lowest frequency SAR currently in use by the geo-science community operates at 440 MHz, and reaches saturation at approximately 100 to 300 tons/ha. It is noted that all references to biomass herein relate to dry biomass. L and C-band systems are less capable, and saturate at still lower biomass densities. Over 80% of the Earth's terrestrial vegetative biomass exists in forest stands with biomass densities above the P-band system's saturation point. Many of the Earth's largest forests have biomass densities between 800 and 2000 tons/ha. Thus, lower frequency radar systems are required for the accurate estimate of terrestrial biomass.
The airborne radars currently being used for the remote sensing of biomass are multi-million dollar SAR systems. Current SAR technology has been developed primarily for high resolution imaging. The sophisticated and expensive SAR systems which have been developed generate data with spatial resolution in the range from 1 to 20 meters. In order to generate data with this high degree of resolution, large bandwidth and high power signals must be transmitted, driving up the cost of the transmitter. In order to control image distortion, the antenna usually must have provisions for motion compensation to remove variations in yaw, pitch and roll of the aircraft. The receiver must capture and digitize the large bandwidth signal, which must then be processed and stored. The data rate out of a SAR receiver is typically 10 to 100 MB per second, requiring state of the art processing equipment to record and process the data.
The overall sophistication of these systems drives the cost. A low cost radar system cannot attempt to compete with a sophisticated SAR system in terms of resolution and swath width. A radar sensor designed for commercial applications needs to be relatively simple and low cost. The spatial resolution requirements for biomass estimation are on the order of 50 to 500 meters, considerably larger than the resolution provided by most operating SARs. The present invention exploits this fact and provides a method and apparatus specifically for biomass estimation that is lower in cost (both in hardware and in the processing and storage costs), lighter, and simpler.
Many studies have been done using airborne or satellite-borne radar systems for the measure of terrestrial biomass. The SAR system is the primary type of radar system that has been used for these experiments. Historically, the primary funding for SAR research and development has been from the Defense Department with requirements for the surveillance, detection, and imaging of small targets such as tanks and trucks. In order to detect and image small targets, a 1-3 meter angular resolution is necessary. To achieve this degree of angular resolution at long ranges, frequencies above 400 MHz are usually required and implemented for imaging SAR systems. As a result, there have been few significant efforts with SAR surveillance radars below 400 MHz.
Frequencies above 10,000 MHz are rapidly attenuated by atmospheric absorption and are not militarily useful at other than very short ranges. These 400-10,000 MHz imaging SAR systems have been used in aircraft and satellites for biomass measurement and have been proven to be beneficial for the measurement of biomass under certain circumstances. There is a wealth of literature available dealing with the measure and estimation of terrestrial biomass with airborne or satellite-borne imaging SAR systems.
However, the trunks of the trees in the great rain forests lie beneath thick canopies of small limbs and leaves. The canopy reflects the majority of radar energy above 200 MHz. Therefore the tree trunks are not observable by microwave frequencies above 400 MHz. Recent studies have shown that the radar returns from systems above 400 MHz cannot estimate terrestrial biomass in a rain forest with densities above 200 tons/hectare. The same studies document that approximately 80% of the global terrestrial biomass lies in stands of trees with densities from 200 to 2000 tons per hectare. Additionally, the design and development of imaging SAR systems to produce 1-3 meter resolution is very complex and expensive. A typical imaging SAR radar system for target surveillance and detection costs in excess of $1M.
A different type of system is a non-imaging airborne radar system which has been developed to create detailed topographic maps. These maps are presented in plan view and give the elevations of the land surveyed with high resolution. A recent non-imaging airborne SAR type system has been successful in demonstrating the measurement of topography with a 1-meter resolution in elevation. For the mapping mission, frequencies from 10 MHz to 10,000 MHz have been used. At 10 MHz, almost all biomass is transparent. This allows the system to be used to create topographic maps of areas covered by dense rain forests. There are many other non-imaging systems and techniques at various frequencies which have been used successfully to create improved topographic maps in heavily wooded areas. All of these types of systems measure topographic elevations, but none are able to provide a direct measure of biomass. Several of these types of systems have been used to infer biomass estimates, but none have the ability to measure the biomass contained in the trunks of large trees directly.
A number of patents exist which generally relate to the technical field of the present invention. For example, the patent to Vickers, U.S. Pat. No. 4,495,500, disclosed a topographic data gathering method which uses a first radar operating at a frequency of around 200 MHz for penetrating foliage, and a second radar operating at a higher frequency of around 400 MHz. Vickers discloses that the data gathered from the radar can be processed in a manner which provides tree height information for use in determining the amount of timber that can be harvested from a particular area. The 200 MHz radar signal goes through most of the foliage and measures the distance to the ground. The second radar at 400 MHz is partially reflected by the foliage and gives a measure of the distance from the system to the approximate top of the trees. Subtracting one measure from the other gives the average approximate height of the trees. In tree farms, the trees are usually uniform in spacing and height, species, age, growth rate, and subsequently biomass. Therefore, in tree farms, where these conditions are met, the biomass of the trees can be correlated with the height of the trees, and an approximation of the amount of timber can be made. Where trees are of different height, species, age, growth rate, and are randomly located, estimates of biomass from the averaged approximate height cannot be accomplished. Thus, Vickers cannot be used to measure rain forests because of the variation in tree height, age, spacing, etc. This system also cannot be used for the measure of biomass in rain forests, because frequencies used are too high to penetrate the rain forest canopies. Vickers measures the distance to the ground and the approximate distance to the average tops of the trees and then calculates the approximate average height. Only average tree height is estimated. Any other details of biomass are not measured and must be inferred or assumed. Vickers discloses primarily a topographic mapping device. The output from the system of Vickers is a topographic map which indicates elevation data in plan view. For the reasons explained above, Vickers does not enable direct measurement of biomass contained in tree trunks in a rain forest.
The patent to Hellsten, U.S. Pat. No. 4,965,852, discloses a method for the radar topographic mapping of an area, and a device for wideband exploration at frequencies below 300 MHz, and preferably between 12.5 and 200 MHz. This system is used to generate high-resolution elevation maps presented in plan view. Hellsten addresses the specific topographic mapping and observes that the system is able to explore and determine the firm ground contour in forested areas. This makes topographic mapping possible in areas where the vegetation is too extensive for photogrammetrical methods to be usable. Hellsten produces high-resolution topographic maps. Because of the low frequencies, which penetrate foliage, the system can be used to make topographic maps in heavily forested areas. Although the system uses low frequencies, it does not make any direct measure of factors related to biomass.
The patent to Schuler, U.S. Pat. No. 5,552,787, discloses a SAR system designed to measure terrain elevation. The disclosure at column 2, lines 14-18 and column 14, lines 23-28 indicates that the invention can be used to provide "topographic information over a wide area so that geophysical parameters, when such information is needed to correct radar studies of soil moisture, biomass density, and crop type may be accurately measured by radar". Schuler produces a wide-area high-resolution topographic map, indicating elevation as a function of location presented in plan view. This detailed elevation map can be used to correct radar studies (from other radar systems) of soil moisture, biomass density, and crop type. There is no disclosure in Schuler of direct measurement of soil, biomass density, or crop type. This system does not measure or otherwise calculate biomass.
The patent to Gail, U.S. Pat. No. 5,313,210, discloses a polarimetric radar signal mapping process for use with an airborne or spaceborne platform. At column 2, lines 40-57, it is disclosed that "low frequencies are useful in penetrating deeply into areas with substantial vegetation." Gail observes that "it would be advantageous for a spaceborne polarimetric radar signal mapping process to be able to use such low frequency signals for biomass or other applications . . . " It is well known that the lower frequencies are more able to penetrate foliage and are useful in mapping areas of dense vegetation. However, Gail provides no explanation of how such measurements might be carried out.
The following U.S. Pat. Nos. also disclose related radar systems: 4,894,659; 5,053,778; 5,335,181; 5,353,030; and 5,489,907. All of these inventions make use of radar systems for the accurate determination of altitude or elevations, for the generation, correction, or simulation of SAR images or topographic maps. None of these Patents disclose a system which enables remote and direct measurement of biomass.
Accordingly, a need exists for an efficient and inexpensive system and method for directly measuring the terrestrial biomass resident in trunks of large trees, crops or the like vegetative elements present on a surface area of interest and concealed to other known detection and measurement methods and devices by a thick canopy of small branches and leaves or the like, and which enables a map to be generated which indicates the biomass of the vegetative elements as a function of location on the map regardless of the height, age, spacing and type of vegetation present.